Well, then brace, because you asked dangerously complex question
Here goes your answers, at least the way I have experienced MMOs in past 3+ years (there are veterans who have played MMOs longer than me, so I am sure they will have their own few cents to add to this "pool"):
The Beginning
MMO experience for any novice begins as if they are playing regular RPG: because you start your first quest and begin leveling. You are in starter zone and most probably you have no idea what expects you. You get shocked when you discover about "Capital city" of your faction, and as you finish discovering every corner in it, you start loving your game more. This generally happens between levels 5-10. In this period, you are doing almost everything solo, no concept of grouping or team work so far. Depending on MMO you are playing, you wake from this "dream"-state once you encounter your first elite mob/boss and get your butt kicked...
First Group Work
Then, again depending on MMO you are playing, you reach level 12 and you discover the concept of dungeons. Dungeons are 4-5 person job, can't be soloed unless your tier is way above its level requirements - at least in ESO's case, you can't enter dungeons too far below your tier, which makes it technically impossible to solo any dungeons ever in this game (as far as I have seen in betas of course). At this point, you realize why group-activity is important in MMOs: you just need help of others, plain and simple.
In some MMOs, as you level, you also get acquainted with certain quests which cannot be done alone: those quests also make you leave solo life. This stage generally continues until max level, until you are done with leveling. In this period, you learn about instanced PvPs (where certain amount of people fight each other in Arena style), dungeons, epic quests which all are done in groups, and you do a lot of initial crafting business to master your art.
Remember, the game you are playing hasn't yet started. Because every MMO starts once you reach level 50 with almost all quests done. Until then, believe it or not, as a muscle you are insignificant to anyone: you are like a student, a potential to become a giant one day and claim your place among guild ranks.
Post Max-Level
Game starts at this point. If you have managed to finish (at least, almost) all quests by the time you reach maximum level, then you really have experienced the RPG side of the game. The story side, the single-player classic gaming side of it - which is quite joyful experience. But as enjoyable as it's been, its old school - MMOs exist to extend the frontiers of gaming, and you are just stepping into that extended world, the world of Massive Multiplayer Online gaming. Welcome!
As you step-in this MMO world, you start hearing about concepts such as "raids", "ranked PvP sessions" etc. You start hearing about how serious these stuff are, and how well prepared you should be for them. This is when you realize how novice (I won't use word "noob"
) you have been during the leveling period. You will discover that you have actually played the game completely wrongly, including your skills. First order of business will be to learn your class, from scratch, from experts, and RESPEC your character.
Then immediately you will feel the shocking terror of word grinding.
You will be told that to be raid-ready, you need to do certain stuff everyday, to gain certain gear, skills etc to come closer for raid-readiness. As a novice, of course that will be something new for you, and you will eagerly start "grinding".
If things go well, if you really enjoy it and get to minimum requirements, both in skill and gear, you will be accepted to your guild's events such as, let's say "raids". Raids are not your regular dungeons where simple boss mechanics suffice and once you kill those bosses you end up being a hero, saving the day and get satisfied with your "glories". The first day you raid, you will die so many times, that if a little bit too pessimistic in nature, you will feel like you are playing the game for the first time. Remember Overlord Draugr's in Labyrinthian, in Skyrim? Well, if by accident you reached that dungeon in low level (like level 15) as I did, you know what I mean - each arrow you eat from that Draugr totally makes your concept of fights change.
Also during your first raid, especially if its a PUG (short for "pick-up-group", meaning raid group created from random people in the game), or an intolerant guild, you will be humiliated almost for every negative thing coming from you: everytime you die, for your lack of skill and gear. And you also will see how (rightfully) annoyed people get, when some raider outright disrespects them, by, let's say going to toilet every 10 minutes, or eating during raid, or just by going AFK (away from keyboard), or just messing up and failing consciously etc.
As you gear-up gradually, beat your raid bosses and get veteran yourself, you will start expecting serious raids, serious people who know how to do the job, as being funny or social won't cut it all the time. As in real life, everything has place and time, and there is place and time for socializing in MMOs, and place and time for serious stuff, like raiding.
Why Guild Bank Rules? Why so serious about stuff in Bank?
Imagine you bring in new guy to the guild, he jokes with you, creates some relations, and one day (it might happen even as late as a month, or a year after you been together) BAM, he cleans out Guild Bank and leaves. As a member you might be frustrated, maybe for your own contributions there that got stolen, or team's. But imagine guild masters' emotions there! Our guilds, as any of our work, are like our babies to us! Whoever meddles with our guilds, outright become our #1 enemies in MMOs. Thus, to protect our work - which is our guild, which is made of people we call "guildies" - we must have certain rules in place. Guild Bank rules exist to serve that purpose. Among and similar to other rules that exist there as well.
Why Disciplined/Strict Raid Regulations? Possibly, seemingly high requirements?
In any MMO, you raid with 20+ people. That means its a social event, which becomes evident especially when you are told to login to VoIP server (like Teamspeak) and hear their voices. That, as in real life, requires certain responsibility: because in any mature guild, people playing are in their late 20s or above. Which means, those people generally are working class, with real-life going on. Which means they have limited time to play and they want to get the most out of that limited time.
Don't they want to socialize? Of course most of them definitely do. But MMOs are games, and games primarily are about gaming. In MMOs, we add social component on top of that and make things fun. But that doesn't mean raid should wipe for countless hours, or days, even weeks on one boss, just because one or two raider failed to help themselves and come prepared. (Un?)Fortunately it doesn't work that way! And it's not realistic anyway (How many failed dungeons have you been to, during betas?).
To that end, to increase the quality of raids and offer our raiders something substantial, which we call a progress, we have to have some rules in place. To a veteran MMO player, those rules are something they pray for in every MMO they participate. But to a new guy, as expected, they might seem like an overkill. With experience you will learn the necessity of those seemingly "high" standards and thank your guild for having and enforcing them, hopefully, without any elitist/intolerant incidents (which I believe are also covered in our Raid Core Guidelines).
Do I have to be part of every event in guild?
Definitely not. In Lodge, we don't force people to play in a way they don't desire to. You are free to do whatever you want to. Socialize as much as you want, sit behind keyboard and chat all day long not doing a single quest - not important really. But the moment you decide to be part of bigger plans, you will have new responsibilities towards your guildies. PvE, PvP or even RP you join will have their own set of rules, regulations, requirements and what not. That is a fact! We can't please everyone, all of us know this fact - but we can try our best to protect everyone's common interests.
Does that make Lodge a hardcore guild?
Not at all. We are casual guild in every respect. Our raiding is semi-hardcore, and definitely not hardcore. In hardcore guilds, you raid every single day for minimum 5-6 hours and even accidental, yet noobish mistakes made are met with utmost rage. We raid 3 days a week, and only afterwork, for 3 hours tops. All we do differently, is to increase the performance of those 3x3 hours and get things done.
ANY MORE QUESTIONS?
Few more answers in regard to your early questions:
What is the difference between playing co-op style in a small informal group of 4-5 friends and a huge guild of 500 members?
Firstly, we don't plan to have that many members - except maybe for popularity of our Guild Store (which should be popular as it will be our in-game Market). Having too big a guild is a nightmare to maintain and since raids require 24 people only, you can't have too many raiders unless you plan to have parallel runs - which add more work to guild management and is an undesirable situation for any guild leader. As it is subscription based game, ESO will have rather higher quality and more dedicated community of players and in time you will find additional friends outside your guild(s) with whom you will do certain small-scale social events, like dungeons. But PUGs are notorious for their failures in raids, unless all PUG players are highly geared people who know tactics of raids very well due to their earlier experiences, with gear enabling them withstand damage, perform better. So long story short, guilds are a must-have, if you plan to experience endgame content in fastest and most delicious way possible, hopefully with zero frustrations.
Is there more to the game?
I hope there is, especially in PvE side of things. PvP is still mystery to me, but owning a keep has its advantages to your guild, so guess we will have to be active in that arena as well. Additionally, more than half of Tamriel is locked for launch, which will be enabled gradually as Zenimax introduces new content to the game. Considering that we will hit max level in currently available zones, the new zones will be top level, epic questing areas. Or maybe they will increase level cap for a few extra levels (50 to 55 maybe, or similar). We will see. But average MMO lifetime for any MMO player is around 2+ years, minimum. It can become as long as 12 years
as we have seen in World of Warcraft.